1. Field of the Invention
The present invention relates to novel biological control agents for control of pink rot of potatoes.
2. Description of the Prior Art
Postharvest diseases of potatoes caused by fungal pathogens are a worldwide concern and can result in significant losses in the quality and quantity of potato tubers during storage, transport and the marketing process (Kotan et al. 2009. Biological control of the potato dry rot caused by Fusarium species using PGPR strains. Biol. Control 50:194-198). The causal agent of pink rot on potatoes, Phytophthora erythroseptica Pethybr., can cause particularly devastating disease effects on potatoes in storage. It was first reported as a soil borne storage disease in the United States in Maine in 1938 (Salas et al. 2000. The effect of wounding, temperature, and inoculum on the development of pink rot of potatoes caused by Phytophthora erythroseptica. Plant Dis. 84:1327-1333; and Wharton and Kirk. 2007. Pink Rot. Extension Bulletin, E-2993, Michigan Potato Diseases), and since then it has become widely distributed throughout North America as well as most potato-growing regions around the world (Taylor et al. 2006. Biological significance of mefenoxam resistance in Phytophthora erythroseptica and its implications for the management of pink rot of potato. Plant Dis. 90:927-934).
P. erythroseptica invades the potato plant through the roots and mainly infects tubers at the stem end through stolons previously infected by germinating oospores or zoospores. Although tubers infected with the pathogen usually are found in wet, low lying areas of fields during harvest [Al-Mughrabi et al. 2007. In-furrow applications of metalaxyl and phosphite for control of pink rot (Phytophthora erythroseptica) of potato in New Brunswick, Canada. Plant Dis. 91:1305-1309)] the disease can also develop in sandy soils without excessive moisture (Wharton and Kirk. 2007, ibid). Tubers can also be infected by P. erythroseptica through lenticels and buds as well as wounds that occur during harvesting. High relative humidity combined with poor air circulation and cool temperatures in storage promote pathogen survival and facilitate the infection of additional tubers (Atallah and Stevenson. 2006. A methodology to detect and quantify five pathogens causing potato tuber decay using real-time quantitative polymerase chain reaction. Phytopathology 96:1037-1045; Benson et al. 2009. Phytophthora erythroseptica (pink rot) development in Russet Norkotah potato grown in buffered hydroponic solutions I. calcium nutrition effects. Am. J. Potato Res. 86:466-471; and Salas et al. 2000, ibid). Pink rot infection is usually associated with secondary infection by anaerobic soft rot bacteria and further losses may occur in storage due to bacterial infection of damaged tissue (Wharton and Kirk. 2007, ibid).
Most potato cultivars commonly grown in North America are susceptible to pink rot (Salas et al. 2003. Assessment of resistance of tubers of potato cultivars to Phytophthora erythroseptica and Pythium ultimum. Plant Dis. 87:91-97). A survey of North American cultivars with fungal disease resistance showed that over 25% of 130 cultivars released have resistance to one or more fungal diseases such as early dying, late blight, early blight, dry rot and black scurf, but notably, resistance to pink rot and silver scurf was absent (Secor and Gudmestad. 1999. Managing fungal diseases of potato. Can. J. Plant Pathol. 21:213-221).
Measures for managing pink rot in the field and in storage includes planting in soils with good water drainage, crop rotation, harvesting tubers at temperatures below 18° C., using high airflows and preventing water condensation in the tuber pile during storage, eliminating diseased tubers, and the timely application of mefenoxam-based fungicides (Miller et al. 2006. Post harvest applications of zoxamide and phosphite for control of potato tuber rots caused by oomycetes at harvest. Am. J. Potato Res. 83:269-278; Salas et al. 2000. ibid; and Secor and Gudmestad. 1999. ibid). Many studies suggest that mefenoxam-resistant isolates of P. erythroseptica are now widespread, which likely explains the failure of these chemicals to consistently control pink rot (Benson et al. 2009. ibid; and Taylor et al. 2006. ibid) and points to the need to develop additional methods for reducing new pink rot infections in storage.
Infections by P. erythroseptica initiated after tuber harvest are difficult to control. Studies using phosphorous acids and various salts in furrow have shown the potential of these materials to reduce pink rot on harvested tubers (Johnson. 2008. Post-harvest applications of phosphorous acid materials for control of Phytophthora infestans and Phytophthora erythroseptica on potatoes. Plant Pathol. 7:50-53; Miller et al. 2006. ibid; and Mills et al. 2005. Salt compounds as control agents of late blight and pink rot of potatoes in storage. Can. J. Plant Pathol. 27:204-209), but additional disease reduction technologies for this purpose are still needed. With growing public interest in reducing chemical pesticide residues in food and the environment, the need to develop new pest management technologies that reduce the use of chemical pesticides is apparent.
Several studies have been conducted on the biological control of economically important soil borne storage diseases of potatoes, including studies where microbial strains from various suppressive soils were found to be active against Fusarium dry rot (Schisler and Slininger. 1994. Selection and performance of bacterial strains for biologically controlling Fusarium dry rot of potatoes incited by Gibberella pulicaris. Plant Dis. 78:251-255; and Schisler et al. 2000. Potato cultivar, pathogen isolate and antagonist cultivation medium influence the efficacy and ranking of bacterial antagonists of Fusarium dry rot. Biocontrol Sci. and Technol. 10: 267-279) and late blight (Hollywood. 2008. Biological Control of Late Blight of Potatoes: in vivo and in vitro evaluation of microbial antagonists against tuber blight, Ph.D. Dissertation, University of London, Biology Department, London, England; and Slininger et al. 2007. Biological control of post-harvest late blight of potatoes. Biocontrol Sci. and Technol. 17:647-663) on tubers in storage. However, research on discovering biological control agents that are specifically targeted against pink rot on stored potato tubers is limited (Schisler et al. 2009. Bacterial antagonists, zoospore inoculum retention time and potato cultivar influence pink rot disease development. Am. J. Potato Res. 86:102-111).
However, despite these and other advances, there is a continuing need for improved biocontrol agents for pink rot.